Abba turned down a $1bn offer to reform in 2000

But the foursome only sang one song, a Swedish happy birthday tune, for a friend's party - and are still ruling out a full comeback.

Bjorn Ulvaeus, Benny Andersson, Agnetha Faltskog and Anni-Frid Lyngstad got on stage in front of 160 guests at the 50th birthday party for Benny and Bjorn's manager Gorel Hanser in 1999.

"I was happy to have them sing - but it was not Abba reforming, it was my friends singing," Ms Hanser told BBC News Online.

"They did sing a birthday song - they were one of many singing that night.

I looked right and saw Agnetha singing beside me again and it felt so odd Bjorn Ulvaeus

"There was a small stage where my children and other people were singing or making a speech."

Ms Hanser was a record company executive during Abba's heyday and became close friends will all four band members.

They last played in public in 1980, but when asked whether they would consider a public comeback, Ms Hanser said: "I don't think they'd bother."

The year after the birthday performance, the group turned down a $1bn (�600m) offer for a reunion tour.

Bjorn told the Daily Mirror newspaper that "it was so strange" to perform together again.

"I was standing on that little stage and I looked right and saw Agnetha singing beside me again and it felt so odd."

'Incredible'

The group refused the $1bn offer because they did not want to let fans down, he said.

"I know it sounds incredible, but all we could see was a disappointed audience.

"How could we live up to what we were? The stress wasn't worth it."

But the four singers had "mellowed with age" and were now "the best of friends", he told the paper. "We just have different lives now."

Abba were the most successful group of the 1970s with hits like Dancing Queen and Mamma Mia, reportedly selling 350 million records around the world.